Dynamic electric brake for a variable speed wind turbine having an exciter machine and a power converter not connected to the grid

ABSTRACT

A variable speed wind turbine having a doubly fed induction generator (DFIG), includes an exciter machine mechanically coupled to the DFIG and a power converter placed between a rotor of the DFIG and the exciter machine. Thus, the power converter is not directly connected to the grid avoiding the introduction of undesired harmonic distortion and achieving a better power quality fed into the utility grid. Moreover, the variable speed wind turbine includes a power control and a pitch regulation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. application Ser. No.11/477,593, filed Jun. 30, 2006 and U.S. Provisional Application No.60/783,029, filed on Mar. 17, 2006, the disclosures of which areincorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the field of variable speed windturbines, and, more particularly, to a variable speed wind turbinecomprising a doubly fed induction generator (DFIG), an exciter machine,an intermediate static converter not connected to the grid, powercontrol and pitch regulation.

2. Description of the Prior Art

In the last few years, wind power generation has increased considerablyworldwide. This growth is widely forecast to continue into the nextdecades, even as the industry and technology have arisen to a maturelevel in this field. As wind farms grow in size and the total base ofinstalled wind capacity continues to increase, the importance ofimproving the quality of power output becomes a challenge of hugeimportance to wind developers and utility customers alike.

Electric power transmission is one process in the delivery ofelectricity to consumers. A power transmission system is often referredto as a “grid”. Transmission companies must meet the challenge ofgetting the maximum reliable capacity from each transmission line.However, due to system stability considerations, the actual capacity maybe less than the physical limit of the line. Thus, good clean sources ofelectrical power are needed to improve system stability.

In most applications, wind turbines generate electric power and feedcurrent into the electric grid. This may cause deviations of the localgrid voltage, such as a change of the steady state voltage level,dynamic voltage variations, flicker, an injection of currents withnon-sinusoidal waveforms (i.e. harmonics), and the like.

These effects can be undesirable for end-user equipment and othergenerators or components connected to the grid, such as transformers. Asthe power capacity increases, an evident need arises for improving thepower quality characteristics of the turbine output. The power qualityimpact of a wind turbine depends on the technology involved with it.Despite this fact, wind turbines manufacturers did not consider thepower quality as a main design feature.

Originally, the first wind turbines were designed to work at a fixedrotational speed. According to this model, the wind turbine's generatoris directly connected to the grid and operates at a determined speed,allowing very minor speed variations. In the case of an asynchronousgenerator, only the slip range of the generator is allowed. The slipbeing the difference in the rotation speed of the rotor as compared tothe rotating magnetic field of the stator. The generator's slip variesslightly with the amount of generated power, and it is therefore notentirely constant. Furthermore, these wind turbines need startingcurrent limitation strategies and reactive energy compensation elementsduring normal operation. Wind turbulence produces a non-desirable torquevariation which is directly transmitted to the wind turbine's drivetrain and, hence, to the active power fed to the electrical grid.

A type of wind turbine that keeps the rotational generator speedproportional to the wind speed, is a variable speed wind turbine. Inorder to obtain the maximum efficiency of the wind turbine, thegenerator rotational speed adapts to the fluctuating wind speed. Thistype of wind turbine includes power electronic converters that areconnected to the grid. Due to this kind of interface, harmonic emissionsfrom the turbine's power electronic converters are fed into the grid.

Presently wind turbines of the variable speed type using powerelectronic converters have become widespread. Examples of this variablespeed wind turbine are described in U.S. Pat. No. 5,083,039, U.S. Pat.No. 5,225,712 or U.S. Published Application 2005/0012339. Theseturbines, based on a full converter system, include a generator, aconverter on the generator side, a DC link Bus, and an active converterconnected to the grid. The variable frequency energy of the generator istransferred to the DC link Bus by the generator side converter, andlater transformed to a fixed frequency by the grid side activeconverter. Some disadvantages are common to all full converter systems.The active switching of the semiconductors of the grid side converterinjects undesirable high frequency harmonics to the grid. To avoid theproblems caused by these harmonics, a number of filters must beinstalled. Furthermore, due to the different impedance values on thegrid and previously existing harmonics, different tuning of the filtersis required according to the characteristics of the wind farm location.

Another variable speed wind turbine is described in the U.S. Pat. No.6,137,187. As shown in FIG. 1, this wind turbine configuration includesa doubly fed induction generator (1), a power converter (4) comprisingan active converter on the rotor side (5), a DC Bus (8), and an activeconverter on the grid side (7). In this configuration, only a minor partof the total power is transferred through the converters (5, 7) to thegrid (9). Power can be delivered to the grid (9) directly by the stator(3), whilst the rotor (2) can absorb or supply power to the grid (9) viathe power converter (4) depending on whether the doubly fed inductiongenerator is in subsynchronous or supersynchronous operation. Variablespeed operation of the rotor has the advantage that many of the fasterpower variations are not transmitted to the network but are smoothed bythe flywheel action of the rotor. However, the use of power electronicconverters (4) connected to the grid (9) causes harmonic distortion ofthe network voltage.

Other documents also describe variable speed wind turbines. For example,U.S. Pat. No. 6,933,625 describes a variable speed system which includesa doubly fed induction generator, a passive grid side rectifier withscalar power control and dependent pitch control. In this case, there isan active converter on the rotor side, a passive grid side rectifier anda switchable power dissipating element connected on the DC link Bus.During supersynchronous operation the energy extracted from the rotor isdissipated in the switchable power dissipating element, reducing theefficiency of the wind turbine; during the operation of the wind turbinein the subsynchronous mode, the energy is rectified by the passive gridside rectifier which causes undesirable low frequency harmonics in thegrid. Thus, complex attenuation filters are required. U.S. Pat. No.6,566,764 and U.S. Pat. No. 6,856,038 describe variable speed windturbines having a matrix converter. Both cases include power electronicconverters connected to the grid, which may cause undesired harmonicvoltages.

All the previously mentioned U.S. patents and other existing solutionson variable speed wind turbines that include power electronics haveconverters connected to the grid. Depending on the technology used onthe converters, there are different ranges of harmonics introduced onthe grid which must be attenuated by using filters, and tuned to thefinal application location, making the systems more expensive and lessreliable.

In view of these problems in the prior art, there is a need to providean improved power solution, which may be applied to variable speed windturbines.

Another undesirable problem, especially in the case of weak grids, isthe reactive power consumption during the synchronization of thegenerator. For example, a synchronization method is described in theU.S. Pat. No. 6,600,240. This method starts connecting the generatorstator to the grid while the power converter is disabled and the rotorhas reached a predefined speed. At this moment, the full magnetizingcurrent is supplied by the grid, which causes a reactive powerconsumption. This reactive power consumption is sometimes not allowed bysome new grid compliance regulations. This patent also describes adisconnection process. The process starts reducing the rotor current anddisabling the rotor converter. In this moment, the reactive magnetizingcurrent is supplied by the grid. To disconnect the generator thecontactor is opened with reactive current, decreasing the operationallife of the contactor. Accordingly, there is a need to provide a methodfor synchronization, connection and disconnection to the grid of thedoubly fed induction generator, which avoids the consumption of reactivepower and increases the lifetime of connecting devices.

Another aspect that determines the power quality injected to the grid isthe control of the generator. One type of control of the generator sideconverter is known as “field orientated control” (FOC). The FOC methodis based on the electrical model and the parameters of the machine. Dueto the dispersion of the machine parameters, the torque can not beaccurately calculated, and additional online adjusting loops arerequired. Moreover, the FOC method that is used introduces delays in theflux position identification when a fault occurs in the grid, making itmore difficult to fulfill the new grid compliance requirements.

In prior art variable speed wind turbines with DFIG configuration,although the stator power remains constant, the rotor power is also fedinto the grid through the power converter. Due to the rotor powerripple, the total power fed into the grid is also rippled, affecting theoutput power quality of the wind turbine.

Variable speed wind turbines, which only use a doubly fed inductiongenerator, cannot use electric braking. As described above, in this kindof configuration, power is delivered to the grid directly by the stator,and a minor part of the total power is transferred from the rotor to thegrid through the converters. When an incidental stop of the wind turbineoccurs, for example during a persistent fault in the grid, thegenerator's power decreases sharply. Only fast non-electrical braking,such as blade pitching, can be applied to stop the wind turbine. Thisoperation mode produces great mechanical strengths in wind turbinecomponents, which may cause premature damages. Thus, there exists a needfor additional braking to prevent this mechanical stress.

The use of high voltage DC link transmission (HVDC) in wind farms isdescribed in Patent No. WO01/25628, which includes a synchronousgenerator as the main generation device. Due to the use of synchronousmachines, the output frequency varies with the wind, so especially atlow wind conditions, the ripple content of the output DC voltage becomeshigh. Furthermore, the output transformer and rectifier must beoversized because they must be able to operate at low frequency.Additional details, such as special construction of the rotor circuitrywith low inductance, are mandatory for the accurate regulation of theoutput power.

SUMMARY OF THE INVENTION

According to one aspect of an exemplary embodiment of the presentinvention, there is provided a variable speed wind turbine with a doublyfed induction generator, having at least one or more blades, one or moregenerators, one or more exciter machines coupled to the drive train, oneor more active power electronic converters joined by a DC link Bus withone of the AC side connected to the rotor circuit of the doubly fedinduction generator, and the other AC side connected to the excitermachine. The invention also describes a power control and a pitchregulation.

According to this aspect of a non-limiting exemplary embodiment of theinvention, power electronics are not connected to the grid. Thus, poweris only delivered to the grid through the stator of the doubly fedinduction generator, avoiding undesired harmonic distortion, andachieving a better power quality to feed into the utility grid.Moreover, the use of complex filters and their tuning according todifferent locations may be avoided, making the system more economicaland reliable.

Another aspect of an embodiment of the present invention is that poweroutput remains constant above rated speed avoiding power fluctuationsdependent on speed changes. Due to the topology of the invention, poweris only delivered to the grid through the stator of the doubly fedinduction generator. Thus, the rotor power ripple is avoided and theoutput power quality of the wind turbine is improved.

Another aspect of an exemplary embodiment of the present inventiondescribes a variable speed wind turbine that uses Grid Flux Orientation(GFO) to accurately control the power injected to the grid. An advantageof this control system is that it does not depend on machine parameters,which may vary significantly, and theoretical machine models, avoidingthe use of additional adjusting loops and achieving a better powerquality fed into the utility grid.

A further aspect of an exemplary embodiment of the present invention isthat the method for synchronization of the doubly fed inductiongenerator avoids the consumption of reactive power during the connectionand disconnection to/from the grid, complying with the new gridcompliance regulations. Moreover, this method may avoid connectioncurrent peaks through connecting devices, increasing the lifetime ofsuch components.

A further aspect of an exemplary embodiment of the present inventionprovides a control method to avoid the “wearing” of the collector of aDC motor when used to drive the pitch movement of the blade and improvesthe lubrication of the bearings of the blades.

Another aspect of an exemplary embodiment of the present invention isthat in the case of an incidental stop of the wind turbine, although adoubly fed induction generator is used, it is possible to apply electricbraking. In the event of an emergency, such as a persistent grid fault,an incidental stop of the wind turbine may happen. Then, the excitermachine is used as generator and power can be transferred from excitermachine to direct current Bus. Then, the electric brake may be activatedand part of the electric power is drained in the rheostat of the chopperhelping the generator to stop progressively and avoiding greatmechanical strengths in wind turbine components.

Another aspect of the present invention is that it can be used for highvoltage DC link transmission (HVDC) in variable speed generationsystems.

According to another aspect, due to the topology of the presentinvention, the output frequency of the AC voltage may be fixed, allowinga smaller dimensioning of required rectifiers and transformers, andreducing the ripple content of the DC output voltage under low windconditions, improving the output power quality.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The incorporated drawings constitute part of one or more embodiments ofthe invention. However, they should not be taken to limit the inventionto the specific embodiment. The invention and its mode of operation willbe more fully understood from the following detailed description whentaken with the incorporated drawings in which:

FIG. 1: Illustrates a conventional variable speed wind turbine systemwith doubly fed induction generator and power converters connected tothe grid.

FIG. 2: Illustrates one implementation of a circuit diagram for avariable speed wind turbine having an exciter machine and a powerconverter not connected to the grid according to one exemplaryembodiment.

FIG. 3: Illustrates a block diagram of a power control and a pitchcontrol for a variable wind speed turbine.

FIG. 4: Illustrates a block diagram of one embodiment of the OptimumPower Tracking Control (OPTC) method.

FIG. 5: Illustrates a block diagram of one embodiment of the GFO and theDoubly Fed Induction Generator's Controller.

FIG. 6: Illustrates a block diagram of one embodiment of the ExciterMachine Controller.

FIG. 7: Illustrates a flow diagram of one embodiment of thesynchronization, connection and disconnection sequence.

FIG. 8: Illustrates a block diagram of one embodiment of the pitchcontrol system.

FIG. 9: Illustrates a block diagram of one embodiment of the voltageregulation mode used during synchronization.

FIG. 10: Illustrates a block diagram of one embodiment of the HVDC windturbine with high voltage generator and rectifier.

FIG. 11: Illustrates a block diagram of one embodiment of the HVDC windturbine with low voltage generator, transformer and rectifiers.

FIG. 12: Illustrates a flow diagram of one embodiment of a method forapplying the dynamic electric brake.

DETAILED DESCRIPTION

A variable speed wind turbine according to various exemplary embodimentsis described below. Several drawings will be referenced only asillustration for the better understanding of the description.Furthermore, the same reference numbers will be used along thedescription referring to the same or like parts.

Overview

Generally, the variable speed wind turbine generator according tovarious exemplary embodiments of the present invention channels theelectrical power generated by the rotor during super synchronousoperation of the doubly fed induction generator, to an exciter machine.The exciter machine then converts this electrical energy back intomechanical rotation energy, which can then be used to further increasethe electrical power generated by the stator that is delivered to thegrid. Electrical power is only delivered to the grid by the stator ofthe DFIG avoiding the delivery of power to the grid through powerconverters. Thus, the quality of the electrical power supplied to thegrid is improved.

Additionally, during sub synchronous operation, when the rotor, insteadof generating electrical power, requires an electrical power source, aportion of the rotational energy generated by the wind is used by theexciter machine to generate the electrical power required by the rotor.

The variable speed wind turbine generator system is broadly shown inFIG. 2. In this embodiment, the variable speed system comprises one ormore rotor blades (201), a rotor hub which is connected to a drivetrain. The drive train mainly comprises a turbine shaft (202), a gearbox(203), and a doubly fed induction generator (205). The stator of thedoubly fed induction generator (210) can be connected to the grid byusing one or more contactors (215). The system also comprises an excitermachine (212) such as an asynchronous machine, a DC machine, asynchronous (e.g. permanent magnet) machine, or a reversible electricalmachine that functions as either a motor or a generator, which ismechanically coupled to the drive train and two active electronic powerconverters (222, 225) joined by a DC link Bus (224) (i.e. a back to backconverter) with one of the AC side connected to the rotor circuit of thedoubly fed induction generator and the other AC side connected to theexciter machine (212). The active power converter (225) which regulatesthe exciter machine is not connected to the grid, such that the activepower converter is isolated from the grid. Alternatively, acycloconverter, a matrix converter or any other kind of bi-directionalconverter may be connected instead of a back to back converter. Thesystem could also comprise an electric brake circuit (231), such a DCchopper, connected to the DC Bus. The converter control unit (CCU) (200)carries out the power regulation of the doubly fed induction generatorand the exciter machine. The system comprises filters such a dV/dtfilter (220) which is connected to the rotor circuit of the doubly fedinduction generator in order to protect it against abrupt voltagevariations produced by the active switches of the power electronicconverter. Furthermore, a dV/dt filter (227) is connected between theelectronic power converter and the exciter machine. In one embodiment, aprotection module (219) against grid faults is connected to the rotor ofthe doubly fed induction generator.

The variable speed wind turbine generator system described in thisembodiment can work below the synchronous speed (i.e. subsynchronous)and above the synchronous speed (i.e. supersynchronous). During thesubsynchronous operation, power flows from the exciter machine (212) tothe rotor (211) of the doubly fed induction generator (205), so theexciter machine (212) acts as a generator. On the other hand, during thesupersynchronous operation, the power flows from the rotor (211) of thedoubly fed induction generator (205) to the exciter machine (212),therefore the exciter machine acts as a motor. The power balance duringthe whole range speed is such that power generated/consumed in theexciter machine (212) is consumed/generated in the rotor (211) of thedoubly fed induction machine, except for the losses in the differentelements.

Due to the topology of the variable speed wind turbine generator systemdescribed, power is only delivered to the grid through the stator (210)of the doubly fed induction generator (205). There is no electronicpower converter connected to the grid. Consequently, undesired harmonicdistortion is avoided and a better power quality to feed into theutility grid is achieved. Moreover, the use of complex filters and theirtuning demands according to different locations is also avoided, makingthe system more economical and reliable.

This topology also allows the use of an electric brake in a doubly fedinduction generator configuration. In case of a wind turbine emergencystop due, for example, to a full blackout of the grid, the stator isdisconnected and power produced by the generator can not be fed into thegrid. However, the exciter machine (212) can be used as generator, andhence power can be transferred from the exciter machine (212) to thedirect current Bus (224). Therefore, part of the electric power isdrained in the rheostat of the chopper. Finally, mechanical oraerodynamic brake, such as blade pitching, is applied to stop the windturbine. This embodiment of the present invention allows the generatorto apply electric brake in a DFIG configuration, helping the windturbine to stop and avoiding great mechanical strengths in wind turbinecomponents, which may cause premature damage.

The variable speed wind turbine control system, as shown in FIG. 3,comprises a general controller (302), power controllers and a pitchregulator. The power set point is calculated by the Optimum PowerTracking Controller (OPTC) (303) based on measured wind speed. This setpoint is sent to the General Controller (302) and hence to DFIGController (300). The power delivered to the grid by the doubly fedinduction generator (205) is controlled by the DFIG Controller (300)making an effective regulation of the total active power and the totalreactive power through the active electronic power converter (222). Thepower electronic control of the doubly fed induction generator (205) isbased on the grid flux orientation (GFO). The exciter machine (212) isregulated by an active electronic power converter (225) and controlledby the Exciter Controller (301). The power transferred to/from theexciter machine (212) is controlled by the active electronic powerconverter, using as main regulation variable the DC Bus voltage level,measured with the DC Bus voltage sensor (223).

The variable speed wind turbine control system also comprises a pitchcontrol system, which is based on the limitation of the demanded powerto the exciter. The Exciter Based Pitch Controller (EBPC) (304)regulates the pitch position of the blades in order to limit aerodynamicpower. The EBPC (304) also provides pitch angle set point for OPTC (303)from exciter's power deviation and by measuring the speed and positionof pitch motor (305). In addition, EBPC (304) comprises a CollectorAnti-Wearing & Lubrication System (CAWLS) in order to protect thecollector of the DC machine used for the pitch movement and improvelubrication of blades bearings.

The topology of the present invention is also suitable for high voltageDC link transmission (HVDC) in variable speed generation systems. Asshown in FIG. 10 and FIG. 11, the DC output can be produced by using ahigh voltage generator with a rectifier (1001), as shown in FIG. 10, orwith a low voltage generator and an additional transformer (1101) withone or more secondaries, as shown in FIG. 11, wherein each secondary isrectified and all of such rectifiers are connected in series or parallelway. Additional connecting devices (1002) and protection devices (1003)are required.

Due to the topology of the present invention, the output frequency ofthe AC voltage can be fixed, allowing a smaller dimensioning of requiredrectifiers and transformers and reducing the ripple content of the DCoutput voltage under low wind conditions, improving the output powerquality.

Furthermore, once the wind turbine starts rotating, all the auxiliarysystems can be fed by the exciter machine (212), notwithstanding theoperation of the main generator, reducing the size of theuninterruptible power supply or of the HVDC to AC converter.

Note that, although grid applications are described, it would beapparent to one skilled in the art that the present invention may alsobe used for other applications such as stand-alone power systems or anyvariable speed energy generation system. For example, such othervariable speed energy generation systems may include power systems basedon wave and tidal energy, geothermal energy, solar energy applications,hydroelectric energy, internal combustion engines, etc.

Optimum Power Tracking Controller (OPTC)

The Optimum Power Tracking Controller (OPTC) (303) adjusts the powerreference for the power control loop, performed by DFIG Controller(300), in order to control generator power. This reference is based onmeasured wind speed as the main regulation variable.

According to this embodiment, a variable speed system wherein a trackingof optimum power coefficient (C_(p)) may be carried out within anoperational speed range. This range is determined by a lower speed limit(ω₀) and an upper speed limit (ω₁) and their correspondent lower powerlimit and upper power limit (P₀ and P₁ respectively).

FIG. 4 illustrates a block diagram of one embodiment for the OptimumPower Tracking Controller (OPTC). The main input of OPTC is the windspeed (u), which is measured by means of one or more anemometers. In oneembodiment, this measurement is filtered (401) to avoid undesiredfrequencies to be amplified through the control system and so that asmooth signal is operated.

OPTC calculates a correspondent power value for each particular windspeed (402). This relationship is determined from the overallcharacteristics of the wind turbine, the rotor head mainly, and itspoints correspond to the maximum aerodynamic efficiency. Thus, C_(p) ismaximised to achieve maximum power output. Obtained power value is inputto a power range limiter (403). This implementation comprises the mainloop.

An auxiliary correction (405) of the main loop is applied to theobtained value to improve the responsiveness of the optimised C_(p)tracking. Doubly fed induction generator optimum speed is worked out(406) from measured and filtered wind speed signal. The rotor optimumspeed (on the low-speed shaft) is the result of dividing the product ofoptimal tip speed ratio (λ) and wind speed (u) by the rotor plane radius(R). Doubly fed induction generator rotational speed is calculated bymultiplying this value by the gearbox ratio. Obtained speed value isinput to a speed range limiter (407). The output of this block iscompared (408) with a pitch corrected speed (PCS), calculated in thePitch Adapted Speed Block (PASB) (410).

Pitch angle reference, minimum pitch angle and measured rotational speedare input to PASB. A gain (413) is applied to the difference betweenfiltered pitch angle set point (β_(ref)) and minimum pitch angle(β_(min)). For the coupling, this term is initialised to zero, beingβ_(ref)=β_(min). Measured rotational speed (ω) is added to calculatesaid corrected speed.

After such correction by PASB (408), a gain (409) is applied to theobtained error providing a ΔP to be added to the previously calculatedpower set point.

Once the obtained power set point has been corrected (404), the value isinput to a power range limiter (415) to ensure that this power referenceis within P₀ and P₁ thresholds. The obtained reference is the power setpoint (SP_P).

A rotational speed surveillance (417) is finally applied to this powerset point. In case PCS is lower than ω₀ (419) a gain or a differentcontroller (420) is applied to such speed difference providing a −ΔP. Onthe other hand, if PCS is higher than ω₁ (422), a gain (423) is appliedto calculated error providing a ΔP, proportional to the speed differenceat the input.

Therefore, above detailed correction is applied to the power set pointSP_P, which, in addition, is input to a power range limiter (424) inorder to ensure that calculated set point does not exceed rated power.Hence, the output of OPTC is the effective power reference SP_Pef to betransmitted to General Controller (302) and hence to DFIG Controller(300) in order to control the doubly fed induction generator power.

Due to Optimum Power Tracking Controller, the output power quality whengenerator speeds are equal or greater than the generator speed at whichrated power occurs is improved. In the prior art variable speed windturbines with a DFIG configuration, although the stator power remainsconstant, the rotor power is also fed to the grid through the powerconverter. Due to the rotor power ripple, the total power fed into thegrid is also rippled, affecting the output power quality of the windturbine. Within the present invention, by using an exciter machine and apower converter not connected to the grid, power is only delivered tothe grid through the stator of the doubly fed induction generator,avoiding ripple and improving the output power quality of the windturbine.

Doubly Fed Induction Generator Controller

The DFIG's stator active power and reactive power control is made by theDoubly Fed Induction Generator's Controller (300). This controlleroffers a good regulation performance and control of the total powerdelivered to the grid. This control is based, as it is explained infurther detail below, on different regulation loops, totally independentfrom the electrical parameters of the machine by using the Grid FluxOrientation (GFO). By measuring with a high accuracy the differentmagnitudes to be regulated, the total power delivered to the grid by thestator (210) of the doubly fed induction generator 205 is perfectlycontrolled, achieving a high quality energy.

The Doubly Fed Induction Generator's Controller (300), illustrated inFIG. 5, is based on the Grid Flux Orientation (GFO) control and fourregulation loops: Two current loops (Irq, rotor current loop (509), andIrd, rotor current loop (510)) and two power loops (Ps, Stator activepower loop (505), and Qs, Stator reactive power loop (506)).

In this exemplary embodiment of present invention, the controller isgoing to regulate the DFIG's stator active power and reactive power byregulating the rotor currents (Av_Ird and Av_Irq) and, consequently, thetotal power delivered to the grid. The power controller operates withthe current and voltage magnitudes referred to a two axes rotatingsystem (d,q), so the different current and voltage measurements carriedout by the system are transformed (514, 517) to the referred rotating(d,q) system.

In one embodiment, by controlling the Av_Ird (rotor current referred toas the ‘d’ axis), the magnetising level of the doubly fed inductiongenerator (205) is fixed, so the reactive power flow direction in themachine is established. Furthermore, the doubly fed induction generator(205) may work as an inductive system, consuming reactive power, or maywork as a capacitive system, generating reactive power. In thisembodiment, the control of the Av_Ird is carried out totally independenton the control of the Av_Irq (rotor current referred to ‘q’ axis). Inanother embodiment, by controlling the Av_Irq, the active powergenerated by the doubly fed induction generator and delivered to thegrid is perfectly controlled.

Accordingly, the DFIG's stator active power loop (507) regulates thestator power (Av_Ps), by receiving a stator power set point (Sp_Pef)from the OPTC (303) and, hence, (Sp_Ps) from the General Controller(302). This loop may be based on a PI controller or a differentcontroller with a more complex structure. The DFIG's stator active powercalculation is described in further detail below. The PI controller(507) output is the rotor current set point (Sp_Irq). The Irq rotorcurrent loop (511) regulates the Av_Irq current with this aforementionedset point. This Irq current loop may be based on a PI controller or adifferent controller with a more complex structure. The regulator outputis the Urq rotor voltage set point (Sp_Urq).

Furthermore, the DFIG's stator reactive power loop (508) regulates thestator reactive power (Av_Qs), receiving a stator reactive power setpoint (Sp_Qs) from the General Controller (302). This Sp_Qs may be basedon a fixed value, SCADA settings or the like. This reactive power loopmay be based on a PI controller or a different controller with a morecomplex structure. The stator reactive power calculation is described infurther detail below. The PI controller (508) output is the Ird rotorcurrent set point (Sp_Ird). The Ird rotor current loop (512) regulatesthe Av_Ird current with this aforementioned set point. This Ird currentloop may be based on a PI controller or a different controller with amore complex structure. The regulator output is the Urd rotor voltageset point (Sp_Urd). In one embodiment, this method allows magnetizing ofthe doubly fed induction generator from the rotor, avoiding reactivepower consumption from the grid. Furthermore, controlling the doubly fedinduction generator magnetising level, and measuring the grid and statorvoltages the system is continuously synchronised to the grid, regardingat every moment the amplitude, the frequency and the angle of the statorvoltages generated by the doubly fed induction generator (205).Connection and disconnection systems will be explained below in furtherdetail.

In one embodiment, the AV_Irq and Av_Ird rotor currents are calculatedreferring the three rotor currents measurement (Ir_L1, Ir_L2, Ir_L3)(121), to a two axes rotational system with a rotational angle (μ−ε)where μ is the grid angle, calculated from the measurement of the threegrid voltages (Vg_L1, Vg_L2, Vg_L3) (217), and ε is the rotor anglemeasured with the position and speed sensor (214).

The Av_Ps and Av_Qs are calculated using Id, Iq, Vd, Vq:

$\begin{matrix}{{Av\_ Ps} = {\frac{3}{2}\left( {{{Vsd} \times {Isd}} + {{Vsq} \times {Isq}}} \right)}} & {{Eq}.\mspace{14mu} 1} \\{{Av\_ Qs} = {\frac{3}{2}\left( {{{Vsq} \times {Isq}} - {{Vsd} \times {Isd}}} \right)}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

where Vsd, Vsq, Isd, Isq are obtained by measuring the three statorvoltages (V_L1, V_L2, V_L3) (216) and the three stator currents (I_L1,I_L2, I_L3) (118), and referring these voltages and currents to a twoaxes rotational system, using the μ rotational angle.

Both current regulator outputs, Sp_Urd and Sp_Urq, are transformed intoa fixed system, using the rotating angle (μ−ε), obtaining the threevoltage references to be imposed in the rotor (211) of the doubly fedinduction generator (205). Block 414 shows the transformation of therotor voltages, from a two axes rotational system to a three phase fixedsystem. In one embodiment, these rotor voltages may be used as referenceto a module for generating the triggering of the active switches of thepower electronic converter (222). Block 415 shows the module wheredifferent PWM techniques may be implemented.

According to this embodiment, an electronic power control system basedon two power loops and two current loops, independent on the machineelectrical parameters, avoids the effects of the electrical parameterdispersion or the theoretical modelling errors in the power regulation.Errors caused by the electrical parameters change because of temperatureoscillations or saturation effects due to the non linearity and areavoided by this method. Thus, a very good quality energy generation isobtained, fulfilling and improving the requirements of the differentnormative. Only different measurements are necessary to make theregulation (I_L1, I_L2, I_L3, V_L1, V_L2, _L3, Ir_L1, Ir_L2, Ir_L3, ε,ω). In one embodiment, the reactive power regulation could be madeindependent of the active power regulation.

Exciter Controller

In one exemplary embodiment, the variable speed system comprises adoubly fed induction generator (205) wherein the rotor (211) isconnected to an electronic power converter (222). This electronic powerconverter is coupled through a DC Bus system (224) to a secondelectronic power converter (225). In one embodiment, this frequencyconverter (power converter) (225) is connected by contactor (228) to theexciter machine (212). The exciter machine, such as an asynchronousmachine, a DC machine or a synchronous (e.g. permanent magnet) machineor a reversible electrical machine, is mechanically coupled to the drivetrain.

Depending on the rotor speed, the power demanded to the exciter machinemay be positive or negative, according to the direction of the rotorenergy flow. During the subsynchronous operation, i.e. below thesynchronous speed, power flows from the exciter machine (212) to therotor (211) of the doubly fed induction generator (205), so that theexciter machine (212) acts as a generator. During the supersynchronousoperation, i.e. above the synchronous speed, the power flows from therotor (211) of the doubly fed induction generator (205) to the excitermachine (212), therefore the exciter machine (212) acts as a motor. Thepower balance during the whole range speed is such that powergenerated/consumed in the exciter machine is consumed/generated in therotor of the doubly fed induction machine, except for the losses in thedifferent elements.

In this embodiment of the present invention, the exciter machine (212)is regulated by the electronic power converter (225) and controlled bythe Exciter Controller (301). The control system of the exciter machine(212) is described below referring to the exciter machine as a permanentmagnet machine. It should be apparent to one skilled in the art thatdifferent type of machines may be used as an exciter machine (212), sothe exciter controller may modified accordingly.

Power transferred to/from the exciter machine (212) is controlled by theelectronic power converter (225), using as main regulation variable theDC Bus voltage level, Av_Ubus. FIG. 6 describes one embodiment of theexciter machine regulation. The Converter Control Unit (200) fixes a DCBus set point voltage Sp_Ubus (605) which may be variable or static. Bymeasuring the DC Bus voltage, the DC Bus voltage set point is regulatedby a PI controller (607) or a different controller with a more complexstructure. This controller establishes the active power to be transferbetween the permanent magnet exciter machine (212) and the DC link Bus(224) in order to keep the DC Bus voltage at the value fixed by theConverter Control Unit (CCU). This active power is determined by theSp_IEq. In one embodiment this Sp_IEq is calculated from two terms:Sp _(—) IEq=Bus voltage regulator (607) output+Decoupling & switching oncompensation (608) output  Eq. 3

where the first term responds to possible Bus oscillations and thesecond term, Iz, is a feed forward term which represents the estimatedcurrent circulating through the Bus. With this type of structure it ispossible to achieve high dynamic power response of the permanent magnetmachine. In one embodiment, the Bus current estimation term does notexist, so the Bus voltage regulator (607) is taking charge of generatingthe effective Sp_IEq demanded to the permanent magnet exciter machine.

In this embodiment, the Sp_IEq is regulated by a PI controller (613) ora different controller with a more complex structure, using the Av_IEqwhich represents the exciter machine active current referred to a twoaxes rotating system. In one embodiment, a permanent magnet machine maybe used, so a field weakening module is required to be able to reducethe machine flux and to have a better power regulation at high speed. Ina permanent magnet machine the stator voltage depends on the rotor speedand on the machine magnet flux. Consequently, above a rotor speed isnecessary to reduce the stator voltage by reducing the flux on themachine.

In one embodiment a field weakening system is implemented, establishinga reactive current set point, Sp_IEd (618) which is going to be demandedto the permanent magnet exciter machine (212). In this way, independenton the rotor speed, the voltage generated by the permanent magnet iscontrolled and placed in the band range regulation capability of theelectronic power converter (225). The Sp_IEd (618) is regulated by a PIcontroller (614) or a different controller with a more complexstructure, using the Av_IEd which represents the exciter machinereactive current referred to as a two axes rotating system.

In one embodiment, the Sp_Id fixes the magnetising level of the machine,and its voltage level. The Sp_IEq fixes the active power injected ordemanded to permanent magnet machine.

In one embodiment, two or three exciter machine phase currents may bemeasured (IExc_L1, IExc_L2, IExc_L3) in order to calculate Av_IEd andAv_IEq. The three currents are initially transformed (601) to a two axesstatic system so IE_sx and IE_sy are obtained. Secondly, these twocurrents are referred (603) to a two axes system which rotates with thepermanent magnet machine total flux, obtaining Av_IEd and Av_IEq. Thiscurrent transformation is made by using the angle μExc, obtained fromthe three or two exciter machine phase voltages which may be measured orestimated (VExc_L1, VExc_L2, VExc_L2). Blocks 602 and 604 show how thepermanent magnet machine flux and voltage absolute values are obtained.

In one embodiment an Effective Voltage calculation module (615) isrequired because the voltage to be generated by the electronic powerconverter (225) must rely on the flux interaction in the permanentmagnet machine due to the effect of current circulation. So, voltage setpoints Sp_UErd and Sp_UErq are calculated (615) from the two PI currentregulators (613, 614) outputs and from Av_IEd, Av_IEq and |VE|.

The two voltage set points, Sp_UErd and SP_Uerq, are transformed (616)into a three axes static system, using the rotating angle μExc. Thus,the voltage references Sp_UE_Rx and Sp_UE_Ry are obtained to be imposedin the stator of the permanent magnet exciter machine (212). In oneembodiment, these voltage set points may be used as references to amodule for generating the triggering of the active switches of the powerelectronic converter (225). Block 617 shows the module where differentPWM techniques may be implemented. In one embodiment, a dV/dt filter orany other kind of filter (227) may be installed between the electronicpower converter (225) and the exciter machine (212).

In one embodiment, the exciter machine (212) may be used to supplyenergy to different elements of the wind turbine, using this machine asan Auxiliary Power Supply. Grid disturbances or faults do not affect thepower electronic converter (225). Consequently, the exciter powerregulation is not affected.

Dynamic Electric Brake

According to another embodiment, a Dynamic Electric Brake (DEB) isprovided that allows the wind turbine to apply an electric brake to stopthe generator. Therefore, mechanical strengths in wind turbinecomponents, which may cause premature damages, may be avoided.

The variable speed wind turbine of present invention comprises a doublyfed induction generator (205) where the rotor (211) is connected to anelectronic power converter (222). This electronic power converter (222)is coupled through a DC Bus system (224) to a second electronic powerconverter (225). This frequency converter (electronic power converter(225)) is connected to the exciter machine (212). The exciter machine,such as an asynchronous machine, a DC machine, a synchronous (e.g.permanent magnet) machine or a reversible electrical machine, ismechanically coupled to the drive train. The system also comprises anelectric brake circuit (231), such a DC chopper, connected to the DCBus.

Within prior art DFIG topologies, if the stator power of the DFIGdecreases abruptly due to a grid fault or a disconnection from the grid,the machine tends to speed up. In the case of a wind turbine operatingat rated power, the machine may suffer an overspeed. Usually, it is notpossible to use electric brake in such moment, because the DFIG's statorpower and, furthermore, the DFIG's rotor power may be too low. However,due to the topology of the present invention, the exciter machine powercan be used to drive an electric brake. In this case, the excitermachine will be used as generator and, hence, power can be transferredfrom the exciter machine to the direct current Bus. Thus, part of theelectric power is drained in the rheostat of the chopper connected tothe DC Bus avoiding overspeed of the generator. In such a way, the windturbine braking does not solely depend on the mechanical brake. In oneembodiment, an electric brake may be used together with mechanicalbrake, allowing the wind turbine to brake progressively, minimizingmechanical strengths, peak torque loads and undesired accelerations. Forinstance, the electric brake may be applied until mechanical oraerodynamic brake is able to take the control of the turbine.

Furthermore, the exciter machine and the DFIG rotor circuitry may beused in tandem with an electric brake circuit to function as an electricbrake to stop or slow the rotation of the generator. In such a case, thebraking is accomplished as follows: when the brake is activated, therheostat is switched on, and the electrical power flows to the rheostat.Said electrical power could flow from the rotor circuit of the DFIGand/or from the exciter machine, according to the power capabilities ofeach. This process does not depend on the sub-synchronous orsuper-synchronous operation mode.

While on super-synchronous operation mode the whole braking power isneeded, at sub-synchronous mode the wind turbine is working at lowspeeds and only a minor part of braking power is needed.

Another application of the Dynamic Electric Brake is when operating athigh wind speeds. If a wind gust occurs when the machine is alreadyoperating near maximum speed, is necessary to brake the machine to avoidstopping due to overspeed.

Within prior art DFIG topologies, it is possible to increase the outputpower from the stator until the pitch of the blades is modified to slowthe wind turbine. This operation system reduces the quality of theoutput power, due to the peaks caused by the wind gust.

Due to the topology of the present invention, it is possible to maintainthe stator output power constant while activating the Dynamic ElectricBrake; by this way, the quality of the output power remains high, andthe speed is reduced until the control of the wind turbine is taken withthe pitch of the blades. As the Power of the Dynamic Electric Brake canbe controlled in a very fast way, accurate control to avoid overspeedcan be performed.

As a result, due to the exciter machine (212), braking power is alwaysavailable. Depending on the exciter power, the exciter converter power,and the rheostat value of the chopper, braking power could reach, in anembodiment, 30% of generator's rated power.

Thus, there is also a maximum braking power (P_(b) _(—) _(MAX))continuously available:P _(b) _(—) _(MAX)=(V _(DC) _(—) _(bus))² /R _(brake)  Eq. 4

wherein V_(DC) _(—) _(bus) is the actual value of DC Bus voltage(Av_U_(bus))

Braking power may be controlled in such a way that when wind turbine isworking at low speeds only a minor part of braking power is needed.However, if wind turbine generator is above rated speed it may benecessary to use the whole braking available power. Thus, a set point ofbraking power (SP_P_(b)) is worked out depending mainly on measurementsof wind speed and generator speed.

In order to control the necessary braking power accurately, a modulationfactor (f_(MOD)) is calculated. This modulation factor is applied to themaximum braking power available in each moment (P_(b) _(—) _(MAX)) toobtain the SP_P_(b).SP _(—) P _(b) =P _(b) _(—) _(MAX) ·f _(MOD)  Eq. 5f _(MOD) =SP _(—) P _(b)·(R _(brake)/(Av _(—) U _(bus))²)  Eq. 6

The modulation factor allows an accurate control of the braking power. Aprogressive electric braking is possible to apply. For example, in anemergency stop of the wind turbine, at the beginning, the whole brakingpower is needed. Once mechanical braking, such as blade pitching, isactivated, it is possible to progressively decrease the electricbraking.

The Dynamic Electric Brake, in this exemplary embodiment, is composed ofa rheostat (resistor, set of resistors or whatever dissipative element)activated by an electronically controllable switch (e.g. an IGBT).Anti-parallel diodes may be also used. DEB is not strictly limited tothe embodiment which has been described. Thus the braking chopper maycomprise elements different from those indicated above.

The Dynamic Electric Brake may be activated in response to variousoperating parameters. In one embodiment, the speed of the shaft 213 orthe turbine shaft (202) may used to activate braking. This speed may bedetermined by the position and speed sensor (214) and may be used toactivate braking when exceeding a threshold. Additionally, accelerationor the change in speed with respect to time may used as an activator. Inthis case, the change in speed over time may be measured by the positionand speed sensor (214) to sense any unusual acceleration. If theacceleration exceeds a threshold, the braking may then be activated.Additionally, many different braking sensing conditions are contemplatedin this exemplary embodiment. Accordingly, the activation of theelectric brake may depend on various braking conditions based on theexciter shaft speed, the exciter shaft acceleration, the DFIG rotorspeed, the DFIG acceleration, the DFIG rotor frequency and sequence, therate of change of the DFIG rotor frequency and sequence, the exciterfrequency, the rate of change of the exciter frequency, the excitervoltage or the rate of change of the exciter voltage.

The operating power, currents and voltages may also be used to activatethe Dynamic Electric Brake. For example, if the rotor (211) currents orexciter machine (212) currents exceed a threshold, the braking may beactivated. Likewise, the rotor (211) and exciter (212) voltages may alsoindicate some sort of operational anomaly. If such an anomaly isdetected, the braking may be activated. Thus, the brake may be activatedwhen currents and voltages of the rotor (211) or exciter (212) exceedthreshold values. Likewise, the frequency of the exciter (212) or rotor(211) currents and voltages may also be used to activate the brake asthey may be indicative of excessive speed or some other sort ofequipment failure.

Accordingly, the electric brake may be activated using the methodillustrated in FIG. 12 as follows. First, a braking condition is sensed(operation 1200). Then, this breaking condition is evaluated todetermine if it exceeds a threshold value, for example, an excessivecurrent, speed, voltage or an excessive rate of change of the current,speed or voltage is compared to some threshold value indicative thatbraking may need to be applied (operation 1201). Based on the brakingcondition and the threshold value exceeded, the braking power iscalculated in operation 1202. After the braking power is calculated, thedynamic electric brake may be activated (operation 1203). The power fromthe activated dynamic brake is drained in a dissipative element inoperation 1204.

With regard to detecting unusual or large accelerations of thegenerator, the change in voltage and current with respect to time, aswell as the change of the frequency of these parameters over time, maybe used to indicate an unusual acceleration. Thus, if such anacceleration is detected, the braking may be activated.

Connection (Enable) Sequence

A connection sequence is provided according to another embodiment. Thisembodiment comprises a doubly fed induction generator (DFIG) (205)coupled to an exciter machine (212) with no power electronic converterconnected to the grid and a connection sequence that allows connectionof the doubly fed induction generator to the grid with no consumption ofreactive energy and no connection current peaks through contactor (215),thus, increasing the lifetime of the contactor (215). FIG. 7 shows theconnection sequence. It would be apparent to one skilled in the art thatthe techniques described here can also be applied if a main circuitbreaker or any other switch, instead of contactor is used to couple thegenerator to the grid.

During normal operation Mode, the turbine is continuously orientingtowards the wind with the use of the yaw motors. When the measuredaverage wind speed is greater than a threshold (in one embodiment 2.5meters per second), if all the rest of required conditions arefulfilled, the blades are moved by the pitch motor to a position thatallows the main rotor to start rotating.

In one embodiment, the initial conditions must be fulfilled beforestarting the connection sequence (701). These conditions involve therotor speed, the state of the rotor contactor (228) and any otherprevious conditions to start the sequence. In one embodiment, once theseconditions are fulfilled the rotor speed must go up to N1 (in oneembodiment, with a 1800 rpm/60 Hz synchronous speed DFIG, the N1 valuemight be 1170 rpm). Once this rotor speed is reached, the exciter sideelectronic power converter (225) is activated in order to regulate theDC Bus voltage level, corresponding to state 702.

In one embodiment, once the DC Bus has reached the VBUS1 level, therotor speed must go up to N2>=N1 (in one embodiment, with a 1800 rpm/60Hz synchronous speed DFIG, the N2 value might be 1260 rpm, and the VBUS1level, with 1700V IGBT, might be 1050V). The DFIG side electronic powerconverter (222) is then switched on (703) in order that voltage throughcontactor (215) comes near 0; This is accomplished by magnetizing thedoubly fed induction generator (205) through the rotor (211) with theelectronic power converter (222), in a way that voltage value, sequence,frequency and other variables are equal in both sides of the contactor(215). When the conditions of voltage amplitude, voltage frequency,voltage angle/delay and some other conditions are fulfilled, thecontactor (215) is closed (704) and stator current is near 0. There isno consumption of energy from the grid by the doubly fed inductiongenerator (205), and possible perturbations on the grid are avoided.

Once this sequence has been fulfilled, power control is activated (705).To allow a smooth connection to the grid, the active power set pointfrom the OPTC, and the reactive power set point from the main controllerare ramped up during the initial moments.

During all the connection sequence, the status of all the involvedelements is monitored in such a way that if an error is detected, thesequence is resumed and an alarm is generated. Depending on the type ofalarm, the sequence can start a predetermined time later, or if theerror is important, one emergency mode is activated in the wind turbinewhich requires human intervention to exit that mode.

The control system used during state 703 for synchronization isdescribed in FIG. 9. A stator voltage regulation is performed. Thestator voltage and the grid voltage are the inputs to the stator voltageregulator (903 and 904), and the output of this regulator is part of arotor current set point in axis d. A current term proportional to themagnetizing current of the generator is added to the output of thevoltage regulator as a feed-forward element. Such current feed-forwardis calculated according to the measured grid voltage, measured gridfrequency and to a K constant that depends on the electrical parametersof the generator. With the addition of this feed-forward term withinblock 905, the synchronization process is sped up. The sum of bothterms, which is the output of block 905, is the rotor current set pointin the “d” axis. During all the synchronization process, the set pointof rotor current in the “q” axis is equal to 0. Both current set points(in the “d” axis and the “q” axis) are the inputs to a currentregulation block (906), wherein they are controlled with PI regulators.The angle used for the conversion of a two axis system (“d” and “q”)into a 3 phase system in block 906, is calculated on the basis of thegrid angle and the mechanical angle in block 907.

Disconnection (Disable) Sequence

A disconnection sequence is provided according to another embodiment ofthe present invention. This embodiment comprises a doubly fed inductiongenerator (DFIG) (205) coupled to an exciter machine (212) with no powerelectronic converter connected to the grid and a disconnection sequencethat allows disconnection of the doubly fed induction generator (205)from the grid without any perturbation related to over-currents orover-voltages on the different elements of the system. Due to theopening of the contactor (215) in near 0 current, the lifetime of thiscontactor is increased and maintenance operations are reduced. It alsoallows a lower rating of the contactor for the same application,compared with other disconnection sequences.

In normal operation of the wind turbine, this sequence is usuallyreached because of absence of wind conditions, but it can be alsoreached in case of excessive wind, local human request, remoteSupervisory Control and Data Acquisition (SCADA) request, a fault in anysubsystem of the wind turbine or any other reason.

In one embodiment, the stator power and stator current must be decreasedwith a ramp in order to have no current in the generator's stator (710).The ramp down time is optimized according to the reason of thedisconnection sequence request. In order to avoid unnecessary mechanicalstress in the wind turbine, the ramp down time is the maximum thatallows a safe operation of the wind turbine. It is evident that rampdown time requirements are not the same for every situation.

Once that state (710) has been fulfilled the main contactor (215) isopened, reaching (711) state. As Active and Reactive Power Set Pointsare 0 prior to opening the contactor (215), the DFIG Controller (300) isinjecting the magnetizing current to have the DFIG stator grid connectedbut without current, so that the opening of the contactor is made withnear 0 current, extending the lifetime of the contactor (115).

When the (711) state is fulfilled the rotor electronic power converter(222) is disabled, corresponding to the (712) state. When the rotorelectronic power converter is disabled, the energy stored in theinductive circuits of the doubly fed induction generator is transferredto the DC link.

Exciter Based Pitch Controller (EBPC)

In this embodiment of the present invention, the variable speed windturbine comprises an Exciter Based Pitch Controller (EBPC). FIG. 8describes one exemplary embodiment of such a pitch control system, whichis based on the limitation of the demanded power to the exciter.

Pitch control system main magnitude is the power of the exciter. Anexciter rated power value (801) is established. An exciter power limiterregulator (804) fixes, from this reference, a blade position set point(Sp_β) depending on the exciter power actual value (802). In oneembodiment, when wind turbine's power output remains below the ratedpower, the Sp_β will take low values (for example between 0° and 2°) andonce the rated power is reached the Sp_β will increase in order to limitthe exciter power.

In one embodiment, the blade pitch position output of 804 is regulatedby a PI position controller (806) or by a different controller with amore complex implementation. The error that is input to the PI positioncontroller is:Error_(—) β=Sp _(—) β−Av_β  Eq. 7

The Av_β is the blade position actual value which is measured by theposition and speed sensor (214). The position regulator output is thepitch speed set point (Sp_n). Blades will move at such speed to reachrequested position.

In one embodiment, the pitch speed output of 806 is regulated by a PIspeed controller (808) or by a different controller with a more compleximplementation. The error that is input to the PI speed controller is:Error_(—) n=Sp _(—) n−Av _(—) n  Eq. 8

The Av_n is the actual value of the blade speed which is measured by aspeed sensor (214). The speed regulator output is the current set pointto be demanded to the DC Motor (305) in order to reach requested speed(Sp_n).

In one embodiment, the current output of 808 is regulated by a PIcurrent controller (810) or by a different controller with a morecomplex implementation. The error that is input to the PI currentcontroller is:Error_(—) I=Sp _(—) I−Av _(—) I  Eq. 9

The Av_I is the actual value of the DC Motor current which is measuredby a current sensor (812). The current controller output is thereference voltage to be imposed in the DC Motor. In one embodiment,these reference voltages may be created through different PWMtechniques, triggering the active switches of the power electronicconverter (811).

In one embodiment, in case of an emergency, pitch motor drive isswitched from EBPC to the Emergency Power Supply (EPS). Therefore,driven motor is directly fed by the EPS (816), through the emergencyrelay (717) until the feathered position is reached (close to 90°). Theblade position switches (818) determine the end of current supply fromthe EPS.

In one embodiment, the drive to move the blade is a DC motor. It wouldbe apparent to those skilled in the art that an AC induction motor or anAC synchronous motor can also be used.

In one embodiment, the drive to move the blade could be a hydraulic,pneumatic or other type of pitch actuator controlled by a servo valvethat integrates the functions (807, 808, 809, 810, 811).

Collector Anti-Wearing & Lubrication System (CAWLS)

In another embodiment of the present invention the variable speed windturbine comprises a pitch control system based on the limitation of thedemanded power to the exciter machine.

In the case that DC motors are used as drives for the pitch movement, aCollector Anti-Wearing & Lubrication System (CAWLS) is applied to avoidfurther harmful effects of keeping a fixed pitch position for a longperiod of time. For instance, premature wearing of collector and brushesof the DC motor due to current passing through the same position can beavoided. Furthermore, lubrication of blades bearings is remarkablyimproved.

Thus, the CAWLS is implemented to avoid premature wearing of thecollector and brushes of the DC motor used as pitch drive and to improvelubrication of blades bearings. In one embodiment, this system is basedon the introduction of a non significant additional set point ofposition or speed, in such a way that the pitch angle is continuouslymoving around the desired position. Pitch angle variation is commandedaccording to a sinusoidal wave reference wherein the amplitude andfrequency are determined from different parameters. Especially, thefrequency should be specified taking into account wind turbine's naturalfrequencies and fatigue considerations. In one embodiment, such asinusoidal wave reference is designed with, for example, a period of oneminute and an amplitude of 0.2°. It would be apparent to those skilledin the art that whatever other wave form, period or amplitude may beapplied. CAWLS implementation does not affect the power production ofthe wind turbine at all, but it does avoid the wearing of the collectorand brushes, and improves their cooling and greasing. CAWLS alsoimproves lubrication of blades bearings.

Furthermore, this system may be used in any kind of pitch drive toimprove the lubrication of blades bearings, increasing the lifetime ofthese components.

Thus, a variable speed wind turbine with a doubly fed inductiongenerator, an exciter machine and an intermediate power converter, whichis not connected to the grid, are disclosed. The invention alsodescribes a power control and a pitch regulation.

Wind power generation has increased considerably worldwide. Growth iswidely forecast to continue into the coming decades, even as theindustry and technology have arisen to a mature level in this field. Aswind farms grow in size and the total base of installed wind capacitycontinues to increase, the importance of improving the quality of poweroutput is a challenge of huge importance.

Many novelties are introduced within the above described exemplaryembodiments of the present invention. An exciter machine is included inthe power system wherein the power converter is isolated from (notconnected to) the grid. Therefore, the invention provides a solution tomost common problems caused by grid connected variable speed windturbines, such as harmonic distortion, flicker and ripple existence indelivered power. Hence, output power quality is remarkable improved.Within these embodiments power output is accurately controlled and, inaddition, it remains constant above rated speed avoiding powerfluctuations dependent on wind speed variations. Indeed, the exemplaryembodiments provide a friendly connection and disconnection methodavoiding reactive power consumption from the grid. Furthermore, powergeneration according to the embodiments of the present invention is lesssensitive to grid disturbances, such as grid faults, and provides abetter performance in stand-alone and weak grids. Thus, the systemillustrated by the exemplary embodiments is especially attractive forthe emerging wind park demands by allowing wind farms to grow in sizeand installed wind capacity, fulfilling the requirements of thedifferent rules and improving power output quality.

In addition, the exemplary embodiments may include some other benefitssuch as: the use of the exciter machine as an auxiliary power supply incase of being a permanent magnet machine, the possibility of generatingpower in medium voltage with a low voltage power converter without powertransformer need, simplification of the electrical components, and theprevention of wear in a DC motor collectors when such a motor type isused to pitch the blades and the improvement of blades bearinglubrication.

Alternative embodiments to the wind turbine system shown in FIG. 2 arealso possible. The exciter machine (212), for example, could beconnected or placed anywhere within the drive train of the wind turbine.Another embodiments including two or more exciter machines are alsofeasible.

From the above description, it will be apparent that the presentinvention described herein provides a novel and advantageous variablespeed wind turbine. Nevertheless, it must be borne in mind thatforegoing detailed description should be considered as exemplary. Thedetails and illustrations provided here are not intended to limit thescope of the invention. Moreover, many modifications and adaptations canbe carried out and equivalents may be substituted for the methods andimplementations described and shown herein. Consequently, the inventionmay be embodied in other different ways without departing from itsessence and scope of the invention and it will be understood that theinvention is not limited by the embodiments described here.

1. A variable speed wind turbine comprising: a rotor including at leastone blade; a drive train coupled to the rotor, the drive train includingat least a doubly fed induction generator (DFIG), said DFIG having atleast a stator connectable to a power grid, at least an exciter machinecoupled to the drive train; and at least a power conversion deviceisolated from the grid and electrically coupled to a rotor of the doublyfed induction generator and to the exciter machine to transferelectrical power between the rotor and the exciter machine; and anelectric braking circuit placed in the power conversion device betweenthe rotor and the exciter machine.
 2. The variable speed wind turbinesystem according to claim 1, wherein the electric braking circuit isconnected to a direct current (DC) bus in the power conversion device.3. The variable speed wind turbine system according to claim 2, whereinthe power conversion device includes a first electronic power converterand a second electronic power converter, wherein the direct current buscouples the first electronic power converter to the second electronicpower converter, wherein the first electronic power converter isconnected to the doubly fed induction generator and the secondelectronic power converter is connected to the exciter machine.
 4. Thevariable speed wind turbine system according to claim 1, wherein theelectric braking circuit dissipates power from at least one of theexciter machine and the rotor circuit of the DFIG.
 5. The variable speedwind turbine system according to claim 1, wherein the electric brakingcircuit includes at least a dissipative element activated by acontrollable switch device.
 6. The variable speed wind turbine systemaccording to claim 5, wherein the dissipative element is a resistor. 7.The variable speed wind turbine system according to claim 1, wherein theelectric braking circuit dissipates the power from at least one of theexciter machine and the rotor circuit of the DFIG, if electric brakingis activated.
 8. The variable speed wind turbine system according toclaim 1, wherein the electric brake is activated in response to asensing of at least one of double fed induction generator shaft speedand doubly fed induction generator shaft acceleration.
 9. The variablespeed wind turbine system according to claim 7, wherein the electricbrake is activated in response to a sensing of at least one of excitershaft speed and exciter shaft acceleration.
 10. The variable speed windturbine system according to claim 7, wherein the electric brake isactivated in response to a sensing of at least one of the DFIG rotorspeed and the DFIG rotor acceleration.
 11. The variable speed windturbine system according to claim 7, wherein the electric brake isactivated in response to a sensing of at least one of a DFIG rotorfrequency and sequence, and a DFIG rotor frequency change rate andsequence.
 12. The variable speed wind turbine system according to claim7, wherein the electric brake is activated in response to a sensing ofat least one of an exciter machine frequency and an exciter machinefrequency change rate.
 13. The variable speed wind turbine systemaccording to claim 7, wherein the electric brake is activated inresponse to a sensing of at least one of an exciter machine voltage andan exciter machine voltage change rate.
 14. A variable speed windturbine system comprising: a first rotor shaft including at least oneblade; a doubly fed induction generator coupled to the first rotor shaftand having a stator connectable to a power grid and having a shaft; anexciter machine coupled to the doubly fed induction generator shaft; apower conversion system isolated from the grid and electricallyconnected to a rotor of the doubly fed induction generator and to theexciter machine to control the doubly fed induction generator; and anelectric braking means for braking at least one of the doubly fedinduction generator shaft and the first rotor shaft.